CalendarToday: tighten up query triggers a bit.

- Add filler_words list which can be stripped from the query before
  trying to parse date strings.
- Anchor datestring_regex so as to allow for only date left in query
  after clean-up.
- Improve guard against empty strings and `undef` in Dates role parsers.

Fixes #661.
Merge pull request #664 from mwmiller/calendar_today

CalendarToday: tighten up query triggers a bit.